When text and or graphics in a multilevel digital image is generated by binary rendering (i.e. only 2 levels of brightness), lines which are neither perfectly horizontal nor vertical are rendered as a succession of vertical and horizontal line segments or pixel patterns. These patterns are commonly known as jaggies or stair steps and degrade the appearance of the text or graphics. It is known that when binary typefonts are displayed on multilevel display devices, the appearance of the type fonts can be improved by placing intermediate gray level pixels at the stair step locations. In the binary laser printer industry, methods to reduce the visibility of "jaggies" are referred to as "Resolution Enhancement Technologies" in reference to Hewlett-Packard's products. This type of defect is also referred to as "aliasing", and the techniques to ameliorate it as "anti-aliasing".
Binary rendering refers to the practice of assigning one of only two possible values to each pixel in an image. Graphics and fonts may be rendered in a binary means for output on a multi-level device, creating the appearance of stair steps. This practice is common among page description language interpreters for color multi-level output devices.
One method to reduce the appearance of stair steps is the application of a digital blur, or low-pass filter, to the digital image. This has the effect of degrading the sharpness of the image without regard for the image content.
There is a need for a method which reduces the appearance of stair steps without degrading the appearance of sharpness in the image without a prior knowledge of the image content.